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Mendel's Dihybrid Cross
Mendel in one of his experimental cross studied the inheritance of seed colour and seed shape. In the first case the two contrasting phenotypes were yellow and green; in the second round and wrinkled. The F1 hybrids of such cross were found to have yellow round seeds. When the F1 hybrids were allowed to cross among themselves they produced four types of seeds in the ratio of 9: 3: 3: 1 (9 yellow round: 3 yellow wrinkled: 3 green round: 1 green wrinkled. The results of such a dihybrid cross has been illustrated in figure.

Thus, beside getting the phenotypic ratio of 3 : 1 of a monohybrid cross, he got the unusual ratio of 9 : 3 : 3 : 1. This kinds of irregularity in the ratio of F2 offsprings of a dihybrid cross was explained by his second law, the law of independent assortment.